This invention relates to radio frequency directional couplers and, more particularly, to a dual directional coupler that generates a true transmitted power measurement.
The performance of cellular telephony and similar duplex wireless communications systems depend strongly on dynamic control of the power transmitted by each base station and each portable terminal. The Federal Communications Commission (FCC), to keep power levels within a safe level, mandates precise control of the power transmitted from the portable terminal.
Prior art methods of measuring transmitted power in a portable terminal use a directional coupler to only sample the incident power at the radiating element. A radiating element may not be perfectly matched to the characteristic impedance of the feedline and a radiating element""s impedance characteristics change as the portable terminal changes its location and proximity to other objects. Also, if the antenna were to be disconnected, the power to the antenna would be measured as being correct even though no power was transmitted. Therefore, it cannot be assumed that the transmitted power be derived with any degree of accuracy from the measured incident power alone.
The present invention overcomes the deficiencies found in the prior art and satisfies the need for a device that can provide an accurate measurement of true transmitted power at the radiating element within a small footprint and a low cost mandated by portable terminals.
The present invention is a dual-directional coupler comprising two detector circuits that generate voltages proportional to the incident power and reflected power. The voltages from the detector circuits are coupled to a differential amplifier. The differential amplifier generates a voltage proportional to the difference between the incident power and reflected power that represents the true transmitted power at the radiating element. The dual-directional coupler is fabricated on a ceramic substrate to facilitate a compact, surface mount implementation. Such a compact coupler can be easily installed in a cellular telephone.